The DIP meter band selection is
similar to most — if not all — DIP
meters in that there is a plug-in coil
for each frequency band and a
single variable capacitor to sweep
the bands. I spent quite a bit of
time selecting the variable
capacitor. They are not as plentiful
as they were 20 years ago.

I decided to use a value of 360
pF since they are the easiest to find
at a reasonable cost. A search on
the Internet yielded several sites
that had at least one variable that
covered the required range. The
company I purchased mine from
has two — one of which has an 8:1
reduction drive which would allow
finer tuning. I used the standard
one (less expensive) for the unit I
built.

The coil assembly for each band is on a small PCB
which is 1” square. I put several of the oscillator
components on these boards so that they could be
customized for each band. Schematic 5 lists the
inductance and capacitance values needed for the
indicated frequency ranges. The circuit designators in the
last column refer to parts in the schematic.

Notice that column 3 is labeled as ΔC. The formula
for frequency can be used to calculate the values in the
table given the low and high frequencies by using it twice
and solving for two unknowns with two formulas:
Since FHi and
FLow are known,
that leaves L, CHi
and CLow as the
unknowns.
However, we can
state that CLow =
CHi+ΔC. Also,
since I have
selected a variable
capacitor with
known minimum
and maximum
capacitance
values, I can
calculate ΔC quite
easily. That now
leaves us with two
formulas and two unknowns: CHi and L. Solving for L using
the two formulas above yields:
with L in µH, C in pF, and F in MHz. After solving this
formula, you can use the value of L to calculate both CHi
and CLow.

The table in Schematic 5 was developed to make use
of this derivation. CHi is the capacitance required for the
high frequency end of the tuning range, and comprises
the minimum capacitance of the tuning capacitor, C1, C3,
and C4 of the tank circuit plus any strays. As it turned out,
I didn’t need to use C1 for any of the bands, but I left it
there in case it is needed to customize the circuit for a